Skeletal muscles are complicated structures made up of at least three contractile fiber types, which differ in functional characteristics and metabolic requirements. To investigate control of microcirculation in skeletal muscle in relation to local metabolism we are engaged in a study of the distribution of capillary blood flow to the different fiber types, and of the influence of blood flow distribution on blood tissue interchange of diffusible materials. The experimental procedures to be used include histological methods for differentiating fiber types and for identifying closed and open capillaries, as well as transport kinetic studies to evaluate capillary transport capacity, exchange and shunt flow. Our goal is an improved understanding of circulation and transport in a tissue which forms the greater part of body mass, and often (during exercise) constitutes the main load on the cardiovascular system.